1. Field of Invention
This invention relates to viscometry and specifically to free rotor viscometry, where there is no mechanical linkage between a rotor that is rotated in a test fluid to measure viscosity and the rotor drive source.
2. Description of Prior Art
Viscosity measurements are useful in many endeavors involving fluid handling and processing. In response, many different methods and devices have been devised to measure viscosity. Many applications need to accurately measure viscosity with a small amount of test fluid. In the area of hazardous or corrosive fluids, there is a need to measure viscosity without exposing the operator to the fluid under test. The available viscosity measurement technologies do not adequately address these needs.
A number of available viscometers consist of a motor directly coupled to a rotor, which rotates in the test fluid. Motor torque or rotation rate is measured from which fluid viscosity may be determined. Brookfield Engineering has a number of popular viscometers that operate this way. One of the drawbacks with this approach is the difficulty in sealing the motor shaft for applications requiring viscosity to be measured at nonstandard pressures or when the viscosity of hazardous or corrosive material is being measured. If the motor drive shaft is sealed, friction between the drive shaft and the seal decreases the accuracy of the viscosity measurement. U.S. Pat. Nos. 2,574,973 and 2,572,693 describe variations of this type of viscometer.
U.S. Pat. No. 5,448,908 describes an approach to measuring the viscosity of fluids with very low viscosities. However, the problem of accurately measuring viscosity in a sealed container remains. In addition, this approach would require a large amount of the test fluid and it would be difficult to assure that the entire volume of test fluid was at the desired temperature during the viscosity measurement.
U.S. Pat. No. 5,798,454 discloses a magnetically suspended rotor disposed in the test fluid and rotated magnetically. The position of the suspended rotor is perturbed laterally and viscosity is calculated from the time required for the rotor to return to its unperturbed position. This invention is designed to be an integral part of a blood pump and measures the viscosity of blood flow. This invention does not measure viscosity of a small sample of test fluid nor is it adaptable to a disposable test chamber.
It is therefore an object of the present invention to provide an accurate viscosity measure.
It is a further object of the present invention to measure fluid viscosity at two or more temperatures.
It is a further object of the present invention to operate in any orientation.
It is a further object of the present invention to measure the viscosity of a test fluid with a small sample of the test fluid.
It is a further object of the present invention to measure the viscosity of a test fluid of low viscosity.
It is a further object of the present invention to make continuous measurements of viscosity.
It is a further object of the present invention to make viscosity measurements on a test fluid sealed within a disposable test chamber.
According to the present invention an apparatus and method for accurate measurement of the viscosity of small fluid samples have been developed. The invention is easily adapted to providing a disposable sealed test chamber to contain hazardous or corrosive test fluids during viscosity measurement.
In the preferred embodiment of the invention, a viscosity tester for fluid comprises a centrally disposed test chamber for containing the fluid, said test chamber having a cylindrical side wall, a circular bottom wall and a circular top wall, the top and bottom walls have perimeters at side wall junctions. A generally cylindrical rotor is disposed in the fluid, the rotor is rotataby supported by bearings on an axis of rotation. The rotor has a plurality of magnetic poles disposed about the axis. A plurality of electromagnets are mounted in positions spaced around the chamber side wall. A plurality of proximity sensors are embedded in the chamber bottom wall, a unique proximity sensor being associated with each of the electromagnets, each proximity sensor producing a sensor signal when a rotor pole is radially aligned with the associated electromagnet. A control system is interconnected with a power supply, the electromagnets and the proximity sensors for selectively energizing the electromagnets sequentially in response to the sensor signals for imposing magnetic forces on the rotor, producing a rotor torque and rotating the rotor within the fluid at a rotation rate. A means for producing an output signal corresponding to the viscosity of the fluid based on rotor torque and rotation rate produces a signal corresponding to fluid viscosity.
In another embodiment of the invention, a viscosity tester for fluid comprises a removable test chamber and a test fixture. The removable test chamber comprises a tube with a closed-end bottom and a circular opening at a top end, the circular opening is sealed by a cap disposed over the opening and engaging the tube, the cap has an inside surface. The removable test chamber contains the fluid and a rotor disposed in the fluid. The rotor is rotatably supported on an axis of rotation on bearings mounted on the inside surface of the cap and on the closed-end bottom of the tube. The rotor further comprises a plurality of magnetic poles disposed about the axis and a position sense magnet. The removable test chamber is supported vertically in the test fixture. The test fixture comprises a plurality of electromagnets, a plurality of proximity sensors, a power supply, a control system, and means for producing an output signal corresponding to fluid viscosity. The plurality of electromagnets are mounted in positions spaced around and proximate to said removable test chamber. The plurality of proximity sensors are mounted in positions spaced around and proximate to said removable test chamber, a unique proximity sensor being associated with each of the electromagnets, each proximity sensor sensitive to the proximity of the rotor position sense magnet and producing a sensor signal when a rotor pole is radially aligned with the associated electromagnet. The control system is interconnected with the power supply, the electromagnets, and the proximity sensors for selectively energizing said electromagnets sequentially, in response to the sensor signals, for imposing magnetic forces on said rotor, producing a rotor torque, and rotating the rotor within the fluid at a rotation rate. The means for producing an output signal corresponding to fluid viscosity based on rotor torque and rotation rate produces a signal corresponding to fluid viscosity.
In accordance with a more general aspect of the invention, an apparatus for measuring the viscosity of a fluid comprises a test chamber for containing the fluid and a rotor disposed in the fluid. The rotor is rotatably supported by a low-friction support means on an axis of rotation. The rotor has a plurality of magnetic poles disposed about the axis. A magnet means produces a magnetic field and imposes a magnetic force on the magnetic poles, producing a rotor torque causing the rotor to rotate in the fluid at a rotation rate. A control means controls one of rotor torque and rotation rate. A sensing means senses one of rotation rate and rotor torque and produces a signal corresponding to the viscosity of the fluid.